Spectroscopy Focus


The Use of a Thermal Desorption System as a Cryogen-Free Method for the Monitoring of Trace Greenhouse Gases in Air


In response to the Kyoto Protocol, an international agreement linked to the United Nations Framework Convention on Climate Change, ‘Clean Development Mechanism’ (CDM) regulations are being enacted in a number of countries to facilitate and control greenhouse gas (GHG) emission trading. Many of the new regulations require the monitoring of bulk greenhouse gases such as carbon dioxide and methane and some require additional consideration of other lower level and more analytically challenging compounds. Examples of this include proposed amendments to the European Emission Trading Scheme Directive 2003/87/EC [1] and Australia’s recent government white paper on a low pollution future [2]. Trace-level greenhouse gases of interest include chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Improved methods for monitoring such compounds in air have recently been reported using sorbent tube or canister-based air sampling methods together with thermal desorption (TD) such as the US Environmental Protection Agency’s (EPA) GC/MS analysis of ‘air toxics’. This work demonstrates detection limits below 100 ppt for all CFCs and HCFCs on the ‘air toxic’ list using an electrically-cooled TD platform with GC/MS running in full scan mode.


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TD offers significantly improved sensitivity when compared with solvent extraction...


Author Details:


Dr David Wevill MRSC, Technical Support Specialist, Markes International Ltd


However, not all trace level greenhouse gases are included on the standard US EPA list of target ‘air toxics’. Perfluorocarbons for example, are a class of long-lived greenhouse gases, the most volatile of which, carbon


tetrafluoride (CF4), has a boiling point of -128°C. CF4 is present in the atmosphere at very low concentrations, but has more than 5,000 times the ‘global warming potential’


(GWP) of CO2 and a half life in the atmosphere of many thousands of years. The extreme volatility of CF4 makes it very difficult to trap/concentrate and measure at low levels.


Similarly, hexafluoroethane (C2F6) has a boiling point of - 78ºC and over 10,000 times the GWP of CO2. Other analytically challenging greenhouse gases, which don’t


appear on the air toxics list include CF3Cl, nitrous oxide (N2


O) and sulphur hexafluoride (SF6) – see Table 1.


Table 1. Greenhouse gases with high GWP, not found in the regular list of US EPA ‘air toxics’.


Example Application


The analytical system used for this study comprised a universal TD (Markes International, UK) and canister interface accessory (with optional nafion dryer), coupled to a GC/MS. Using this TD system, controlled volumes of air/gas were transferred to an electrically-cooled, sorbent- packed focusing trap inside the platform. A combination of Carbograph 1TD™


and Carboxen™ 1003 sorbents held at a


trapping temperature of - 30ºC was found to work best, offering quantitative recovery and efficient release of target compounds. This trap is also compatible with analysis of conventional air toxics (US EPA Methods TO-15 or TO-17).


Not all of these ultra-volatile GHGs are readily available.


CF3Cl, for example, is banned in many countries and cannot be obtained as a standard. It was therefore decided to evaluate the applicability of the same cryogen-free TD GC/MS technology used for air toxics monitoring for the


most challenging ultra-volatile GHG species (CF4, C2F6, SF6 and N2O). If successful, this would demonstrate that such a monitoring system could be used for both ultra-volatile


GHGs plus higher boiling CFC & HCFC air toxics and, by extrapolation, any compound in between.


This article will discuss the performance of TD when compared with traditional techniques and will present an example application of TD for air monitoring.


Monitoring Techniques Use of pumped sampling onto glass tubes packed with


charcoal, followed by carbon disulphide (CS2) extraction and gas chromatography (GC) analysis, was developed as an air monitoring method for vapour-phase organic compounds (VOCs) in the 1970s. The approach is still used today for some personal exposure assessment (occupational hygiene) applications and stack emission testing, but is fundamentally limited with respect to detection limits. Thermal desorption (TD), such as the technology offered by Markes International, is a complementary gas extraction technique whereby sorbent tubes are heated in a flow of carrier gas. Trapped vapours desorb from the sample tubes into the gas stream and are transferred, via a refocusing device, into the GC(MS) analyser.


TD offers significantly improved sensitivity when compared with solvent extraction and has now almost universally


superseded charcoal/CS2 for environmental (ambient and indoor) air monitoring. Steady reductions in exposure limit


levels [3] and new restrictions on chemicals such as CS2, in Europe and elsewhere, have also led to increased use of thermal desorption for occupational hygiene, i.e. for exposure assessment in the workplace. The most recent international standard methods for thermal desorption include workplace air monitoring in the scope [4,5].


TD


Instrument configuration: universal thermal desorption platform + canister interface accessory and nafion dryer accessory


Cold trap: Greenhouse gas trap (part no U-T16GHG-2S)


Sampling time (flow rate): Between 2.5 min (10 ml/min) and 20 min (50 ml/min)


Post sampling line purge (flow rate): 1 min (20 ml/min)


Pre-trap fire purge (flow rate): 1 min (20 ml/min)


Trap low: -30°C Trap heating rate: 40°C/s Trap high: 300°C Trap high time: 3 min Split: Splitless Flow path: 120°C


Once the process of transferring vapours to the focusing trap was completed, the flow of carrier gas was reversed and the trap heated rapidly (up to ~100ºC/sec). Retained compounds were desorbed into the carrier gas stream and transferred/injected into the GC analytical column. Desorption of a TD platform focusing trap is so efficient that splitless analysis is possible without significant peak broadening, for example, all of the retained compounds may be transferred to the analytical column in a narrow band of vapour, ensuring optimum sensitivity.


Analytical Conditions


A custom gas standard containing CF4, C2F6, N2O and SF6 at 1 ppm (balance nitrogen) was used for the experiments.


Detection limits offered by thermal desorption methods are typically at least 1000 times higher than equivalent solvent extraction methods facilitating ambient monitoring at ppt/ppb levels as well as higher ppm (and %-level)


concentrations. By comparison, charcoal/CS2 methods are invariably limited to concentrations above 0.1 - 1 ppm.


The trend towards thermal desorption for all air monitoring applications (workplace, indoor and ambient air) has been further encouraged by recent TD technical developments; the latest commercial thermal desorbers now allow quantitative re-collection of split flow (both tube and trap desorption split flow) for repeat analysis. This overcomes the one-shot limitation of traditional TD methods and simplifies method and data validation.


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